This invention relates to a process for the production of a magnetic recording medium by electroless plating and more particularly, it is concerned with a novel process for the production of a magnetic recording medium having improved magnetic properties by electroless plating.
Thin films of ferromagnetic metals formed by electroplating, electroless plating, sputtering, vacuum vapor deposition or ion plating have lately been noticed as a binder-free, so called non-binder type magnetic recording medium in place of the magnetic recording media of the prior art in which a powdered magnetic material of .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, CrO.sub.2 or ferromagnetic alloy powders is dispersed in an organic binder and coated. It has been proposed theoretically and experimentally to raise the coercive force as well as to make the thickness thinner as one requirement for magnetic recording media for high density recording and it has been expected that such a non-binder type magnetic recording medium has great possibilities, which can readily be made thinner by a factor of 10 than coated type magnetic recording media and has a very large saturated magnetization.
The electroless plating method has the advantage that a magnetic layer can be formed on, in particular, a nonconductive substrate with a good adhesiveness and a magnetic layer having excellent magnetic properties can uniformly be formed. In this specification, the electroless plating means a chemical reduction plating wherein a metal ion to be plated is reduced with a reducing agent in a plating solution and deposited on a substrate under metallic state. In order to start and accelerate this reduction reaction on the surface of the substrate only, it is necessary to give a catalytic activity to the surface of the substrate by a pretreatment. Where a plastic substrate is subjected to magnetic plating, for example, various steps of defatting - etching - substrate surface activating - electroless magnetic plating have commonly been carried as disclosed in U.S. Pat. Nos. 3,245,826 and 3,353,986. The steps of defatting and etching are carried out so as to form a magnetic plating layer uniformly on a substrate with a good adhesiveness and both steps may be accomplished by one solution. For example, an aqueous solution of sodium hydroxide or mixed solution of sulfuric acid and a bichromate is widely used as a defatting and etching solution. The usual method for the substrate surface activation treatment consists in sensitization using a sensitizer consisting of a hydrochloricacidic solution of SnCl.sub.2 and subsequent activation using an activator containing a noble metal ion such as Pd, Au or Ag, as described in U.S. Pat. No. 2,702,253. On the surface of the substrate thus treated are bonded fine particles of Pd, Au or Ag capable of acting as a catalyst of plating reaction. The other methods for the substrate surface activation treatment, as described in U.S. Pat. Nos. 3,011,920 and 3,532,516, have also been put to practical use, which consist in a treatment with a Pd-Sn sol (catalyst treatment) and a subsequent treatment with an accelerator.
In addition, there are special methods for the substrate surface activation treatment such as by causing fine particles of catalytic metal to adhere to the surface of a substrate through vapor deposition, by dispersing fine particles of catalytic metal in an organic binder and coating onto the surface of a substrate and by providing a layer containing a salt of catalytic metal on the surface of a substrate and reducing the salt with a reducing agent to the catalytic metal. As an electroless magnetic plating bath to obtain a high remanence magnetization and coercive force are well known baths containing cobalt ion or cobalt ion plus nickel ion as a magnetic metal ion and hypophosphite ion as a reducing agent (U.S. Pat. Nos. 3,116,159 and 3,219,471). It is widely believed that the electroless plated magnetic film prepared from this plating bath contains a small amount of P deposited at the grain boundary of Co or Co-Ni to thus give a high coercive force.
It has been reported that the coercive force of a Co-P or Co-Ni-P magnetic film made by the electroless plating method of the prior art exceeds 300 oersteds and sometimes reaches 1500 oersteds. According to our experiments however, the coercive force depends on the thickness of a plated magnetic film and increases with the decrease of the film thickness and, in the case of a Co-P film of the prior art, for example, only a coercive force of at most 700 oersteds is obtained when the film is thick enough to give a sufficient out-put as a magnetic recording medium, for example, 0.15 microns thick.